AKNOLD— THE PALEONTOLOGY AND STKATIGKAPHY OF SAN PEDRO. 113 



I 



[S. D.] Pecten (Chlamys) opuntia Dull. 



Pecten (Chlamys) opuntia Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1898, p. 707, 

 PL XXIX, fig. 6. 



Shell of medium size, allied to P. hericeus var. navarchus Dall, from which it differs by its 

 smaller and not fasciculated radial ribs, more elongated anterior ear, more densely, radially costate 

 posterior ear, small size when adult, and by a tendency to be suddenly contracted at the basal margin 

 on the completion of growth. 



Dimetisions.— Long. 32.5 mm.; alt. 35 mm. 



This species is distinguishable from tlie allied species by the large number 

 (forty or fifty) of unfasciculated imbricated ribs, which are of about equal prominence. 



Found in the Pliocene at Pacific Beach, near San Diego, and in the Pliocene 

 at Packard's Hill, Santa Barbara. 



Pliocene. — Santa Barbara (Arnold): San Diego (Hemphill; Hamlin; Arnold). 



Section Plagioctenium Dull. 



Shell thin, orbicular, with subequal inflated valves, usually equilateral, with uniform, well- 

 marked radial, not dichotomous ribs; the concentric sculpture in looped lamellae; the ribs strong, 

 frequently smooth above; the submargins impressed below the subequal auricles; the valves well 

 inflated, with a tendency to oblique growth in the adult. 



Type, Pecten ventricosus Sowerby. 



34. Pecten (Plagioctenium) newsomi, sp. nov. 



Plate XI, Figs. 1 and la. 



Shell of medium size, ovate-triangular, compressed, equilateral, rather thin; twenty-two 

 stout, smooth-topped, squarish ribs, with channeled interspaces of same width as ribs ; interspaces 

 and sides of ribs showing sharp imbricating lines of growth; ears of left valve subequal, with subacute 

 corners; surface of ears radially striated, and showing sharp incremental lines. 



Dimeiisions. — Long. 24 mm.; alt. 26 mm.; diam. 8 mm.; hinge 16 mm.; angle of dorsal 

 margins 87 degrees. 



This variety is distinguishable from P. venlricosu.^ by the much smaller angle 

 (87 degrees) made by the dorsal margins of the disk, that of a typical P. ventricosus 

 being about 110 degrees. P. neivsomi is also very much less ventricose than the 

 latter, and has much narrower ridges, wider interspaces, and a thinner shell; distin- 

 guishable from P. subventricosns by much smaller angle formed by dorsal margins, 

 and by narrower ridges. It resembles P. subventricosns in degree of convexity. This 

 species is named in honor of Dr. John F. Newsom, of Leland Stanford Junior 

 University. 



Rare in upper San Pedro series of San Pedro and Los Cei-ritos. The speci- 

 men figured is the type, which is from the upper San Pedro series at Los Cerritos, 

 and is now in the United States National Museum. 



Pleistocene. — San Pedro (Arnold). 



( 15 ) December 26, 1902. 



